The performance of a skeletal muscle is determined by the interaction of the intrinsic properties of the muscle with the mechanical properties of the system to which it is linked. These properties are complex and predictive equations are limited in some respects by the lack of empirical data on the performance of muscle under loading conditions that replicate those found during movement. The incidence of work and sports related injuries of the musculoskeletal system attests to the fact the mechanical safety factors are sometimes exceeded during normal movement. The proposed research focuses on the influence of the length trajectory on muscle performance and the relation of the resulting force-velocity trajectory to the intrinsic properties of the muscles. The specific aims of the project are: Aim 1: Identify the key muscle properties that have been modified by evolution to allow similar muscles to operate effectively at very different frequencies (or the same frequency at different temperatures). Aim 2: Identify the influences of length trajectory on the mechanical performance and efficiency of muscle. Aim 3: Examine the role that connective tissue elements have in modifying the length trajectory of muscle fibers to enhance performance and prevent damage. Several animal models will be used that were chosen for their suitability in addressing questions that relate to these specific aims. Studies of the adductor muscle in scallops will allow the performance a single skeletal muscle to be measured both in vivo and in vitro. In vivo recordings in scallops use simultaneous high resolution pressure and flow measurements to estimate power output. The in vivo function of frog muscle will be measured by recording length (sonomicrometry and high- speed video) and electromyographic activity. Similar techniques will be used to monitor the function of the leg muscles in running birds. This avian model will be used to contrast the function of muscles that are expected to be stretched while active (eccentric contraction) with those muscles expected to shorten while active (concentric contraction). In vitro muscle performance and efficiency will be measured in contractile cycles that replicate the conditions in vivo. In vitro measurements of tendon strain under dynamic loading will be made to clarify the role of these elements in optimizing muscle performance.